The epidermis of terrestrial vertebrates has evolved such as to form an effective barrier to water loss, a factor essential to survival in a dry environment. It has been known for some decades that this barrier role is provided by a combination of lipid compounds, such as free fatty acids, cholesterol and its derivatives, and numerous ceramide compounds. The latter are extremely hydrophobic molecules, the physicochemical arrangement of which in the intercellular spaces is closely dependent on their molecular structures and on the interactions which they can exchange with proteins. Thus it is that some ceramides are bonded via covalent bonds to the proteins of the membrane of corneocytes. These bonded ceramides provide an effective bond between the other lipids and the proteins, allowing better physicochemical stability of the combination. This concept of bonded lipids (ceramides or fatty acids) is also involved in the development of skin appendages, such as the hair and nails.
The diversity and the complexity of ceramide structures are such that, despite research undertaken for some twenty years, knowledge of the subject is far from being satisfactory.